An air/fuel mixture for an internal combustion engine is delivered to the engine intake manifold through an air/fuel mixture flow passage formed in a throttle body. It is conventional design practice in the case of an engine having an air/fuel mixture carburetor to provide a throttle plate situated in the throttle body on the downstream side of a carburetor venturi. Fuel distribution to the intake air is developed by reason of the venture pressure. In the case of an internal combustion engine having a fuel injection system, the throttle plate would be located in the throttle body in the airflow path between the air intake duct and the engine air intake manifold. The flow of air or the flow of an air/fuel mixture through the throttle body is controlled by the throttle plate as the position of the throttle plate is adjusted by a throttle valve actuator. The actuator may be in the form of a driver-operated linkage or a throttle controller such as an electrical stepper motor.
The throttle plate may be mounted on a throttle shaft that extends through the throttle body passage, hereinafter referred to as an airflow passage. Spaced bearings in the throttle body rotatably support the throttle shaft in a straddle mount arrangement. An example of a throttle body having a throttle plate of this kind may be seen by referring to U.S. Pat. Nos. 5,492,097 and 1,841,695. Each of these prior art throttle valve assemblies includes a circular valve plate situated in a cylindrical airflow passage in the throttle body.
One problem associated with a throttle valve assembly of the kind shown in the '097 and '695 patents is a tendency of the valve plate to frictionally engage the wall of the airflow passage, thereby creating an undesirable binding or sticking of the valve plate which prevents normal throttle valve adjustments. Valve plate sticking is more prevalent when the valve plate assumes a closed throttle position.
Attempts have been made to overcome the valve plate sticking problem by providing for a lost motion connection between the throttle plate shaft and the throttle plate itself in instances when the throttle valve plate shaft is journalled in the throttle body in a cantilever fashion. Such an arrangement is shown in prior art patent U.S. Pat. No. 5,311,849, where a pin-and-slot connection is provided between a cantilever-mounted throttle shaft and a throttle plate so that any binding tendency of the throttle plate is reduced. The cantilever throttle plate mounting arrangement of the '849 patent provides for a floating movement of the throttle plate relative to a relatively stable bearing mount for the cantilever throttle shaft. The design does not make specific provision for allowing flexure of the throttle plate due to a pressure differential that would exist across the throttle plate when the throttle plate assumes a closed position. Neither does it make provision for reducing to a minimum leakage of air through the lost motion connection with the throttle is closed.